Induction heating systems such as induction cooktops can be used to heat cooking utensils by magnetic induction. A resonant power inverter can be used to supply a chopped DC power signal through a heating coil. This can generate a magnetic field, which can be magnetically coupled to a conductive object or vessel, such as a pan, placed over the heating coil. The magnetic field can generate eddy currents in the vessel, causing the vessel to heat.
A typical resonant power inverter circuit is illustrated in FIG. 1. As shown, the induction heating coil 114 can receive a power signal 101 that is supplied through a resonant power inverter, referred to herein as a resonant inverter module 112. The resonant inverter module 112 can be generally configured to generate a high frequency power signal from AC power source 108 at a desired operating frequency to the induction heating coil 114. The load of the resonant inverter module 112 can generally include the induction heating coil 114 and any object or vessel that is present on the induction heating coil 114. The object or vessel on the induction heating coil 114, such as for example a pan, will be generally referred to herein as a vessel.
The resonant inverter module 112 can be coupled to AC power source 108. The resonant inverter module 112 can be provided with switching elements Q1 and Q2, which can provide power to the load, including the induction heating coil 114 and any vessel or object thereon. The direction A, B of the current flow through the induction heating coil 114 can be controlled by the switching of switching elements Q1 and Q2. Switching unit 130 can provide the controlled switching of the switching elements Q1, Q2 based on a switching control signal provided from controller 120. In typical known applications, controller 120 can be configured to control switching unit 130 based on signals from a current transducer or current transformer 110.
Switching elements Q1 and Q2 can be insulated-gate bipolar transistors (IGBTs) and the switching unit 130 can be a Pulse Width Modulation (PWM) controlled half bridge gate driver integrated circuit. In alternate embodiments, any suitable switching elements can be used, other than IGBTs. Snubber capacitors C2, C3 and resonant capacitors C4, C5 can be connected between a positive power terminal and a negative power terminal to successively resonate with the induction heating coil 114. The induction heating coil 114 can be connected between the switching elements Q1, Q2 and can induce an eddy current in the vessel (not shown) located on or near the induction heating coil 114. In particular, the generated resonant currents can induce a magnetic field coupled to the vessel, inducing eddy currents in the vessel. The eddy currents can heat the vessel on the induction heating coil 114 as is generally understood in the art.
The resonant inverter module 112 can power the induction heating coil 114 with high frequency current. The switching of the switching elements Q1 and Q2 by switching unit 130 can control the direction A, B and frequency of this current. In one embodiment, this switching can occur at a switching frequency in a range that is between approximately 20 kHz to 50 kHz. When the cycle of the switching control signal from the switching unit 130 is at a high state, switching element Q1 can be switched ON and switching element Q2 can be switched OFF. When the cycle of the switching control signal is at a low state, switching element Q2 can be switched ON and switching element Q1 can be switched OFF. When switching element Q1 is triggered on, a positive voltage is applied to the coil and the current of the power signal 101 flows through the induction heating coil 114 in the direction of B initially and then transitions to the A direction. When switching element Q2 is triggered on, a negative voltage is applied to the coil and the current of the power signal 101 flows through the induction heating coil 114 in direction of A initially and then transitions to the B direction.
If switching element Q1 is turned on and switching element Q2 is turned off, the resonance capacitor C5 and the induction coil 114 (including any vessel thereon) can form a resonant circuit. If the switching element Q1 is turned off and switching element Q2 is turned on, the resonance capacitor C4 and the induction coil 114 (including any vessel thereon) can form the resonant circuit.
To properly drive an induction coil using a resonant power inverter, such as the resonant power inverter depicted in FIG. 1, it is important to have an accurate assessment of the resonant frequency of the resonant power inverter being used to drive the induction coil. In particular, the output power of the induction coil is a function of the input, the coil inductance, vessel resistance and resonant frequency of the system. The closer the system is driven to resonant frequency, the more power can be delivered to the system. Maximum output can occur at resonance and subsequently lower power levels can be driven away from resonance accordingly.
One drawback of operating an induction heating system using a resonant power inverter circuit such as the circuit illustrated in FIG. 1 is that the switching elements can experience a “hard” switch-off. For example, FIG. 2A provides an exemplary graphical depiction of current and voltage levels associated with a switching element and an induction heating coil of a typical known resonant power inverter circuit. In particular, FIG. 2A shows a coil current 202, a switching element current 204, and a switching element voltage 206. For example, coil current 202 can be the current flowing through induction heating coil 114 of FIG. 1, switching element current 204 can be the current flowing through switching element Q1 of FIG. 1, and switching element voltage 206 can be the voltage across switching element Q1 of FIG. 1.
When resonant inverter module 112 is operated in the fashion discussed above, switching elements Q1 and Q2 switch on and off when coil current is at its peak amplitude. For example, as depicted in FIG. 2A, switching element Q1 is switched off at time t, when coil current 202 is at its peak amplitude. Switching element Q2 (voltage and current not depicted) will then be switched on. In such fashion, the voltage across the induction heating coil can be reversed. However, when switching element Q1 is switched off at time t, switching element Q1 can experience a switching power loss. Such switching loss can be generally proportional to the corresponding coil current. Thus, when the peak amplitude of coil current 202 is relatively high, the resulting switching power loss can exceed the switching element's safe operating area and the switching element can be damaged.
Excessive switching power loss is especially problematic in the instance in which a vessel that is magnetically coupled to the induction heating coil is removed or otherwise shifted away from the induction heating coil. For example, FIG. 2B provides an exemplary graphical depiction of peak coil current levels versus operating frequency of an induction heating coil with and without an associated vessel. In particular, plot 208 depicts peak coil current versus operating frequency for an induction heating coil with an associated vessel. As shown in FIG. 2B, peak coil current for an induction heating coil with an associated vessel can be maximized at resonance frequency 210. Similarly, plot 212 depicts peak coil current versus operating frequency for an induction heating coil without an associated vessel. Plot 212 can reach a maximum peak coil current at resonance frequency 214.
Removing or otherwise shifting the vessel away from the induction coil can result in a reduction in peak coil current and, therefore, a reduction in power output. As an example, with reference to FIG. 2B, removing the vessel from the induction heating coil can cause the peak coil current to shift from plot 208 to plot 212, which can correspond to a decrease in peak coil current and power output at frequencies above resonance frequency 210. In response, the induction heating system can decrease the operating frequency in an attempt to maintain a target or desired power output. Such decrease in operating frequency increases peak coil current and can result in an increased switching power loss experienced by a switching element. However, if the operating frequency is driven too low, the switching power loss can increase to an excessive, damaging amount.
Thus, systems and methods for protecting switching elements in an induction heating system are desirable.